Genic Variation in Hybridizing Populations of Gophers (Genus Thomomys)

Abstract

Patton, James L. (Museum of Vertebrate Zoology, University of California, Berkeley 94720), Robert K. Selander (Department of Zoology, University of Texas, Austin 78712), and Michael H. Smith (Savannah River Ecology Laboratory, Aiken, South Carolina 29801). 1972. Genic variation in hybridizing populations of gophers (genus Thomomys). Syst. Zool. 21:263-270.-Allelic variation at 27 structural gene loci was studied electrophoretically in five populations of Thomomys bottae and one of T. umbrinus in Arizona. The status of eight individuals identified karyotypically as F1 hybrids was confirmed by analysis of their genotypes at several marker loci. Lack of genic introgression between populations of the two species, despite hybridization in a zone of contact in the Patagonia Mountains, presumably results from hybrid sterility caused by genic and/or chromosomal imbalance. Levels of genic heterozygosity (H) are moderate to high in both species (T. bottae, H .071 [.043-.106]; T._umbrinus, H = .031). Genic similarity among populations of T. bottae is also high (S = .933 [.90-.96] ), suggesting either effective gene flow or strong, uniform selection in the populations sampled. [Allozymes; electrophoresis; hybridization; Thomomys.] The analysis of genetic structure in natural populations has become increasingly feasible in recent years through application of electrophoretic techniques for the demonstration of polymorphic variation in enzymes and other proteins encoded by structural gene loci. The present investigation was undertaken to provide insight into two sets of interrelated problems relating to the genetic structure of populations of pocket gophers. We were primarily interested in determining the extent, if any, of genic introgression between populations of Thomomys bottae and T. umbrinus hybridizing in southern Arizona. It has previously been established on karyotypic (Patton and Dingman, 1968) and morphologic (Hoffmeister, 1969) grounds that these species hybridize at a single point of parapatric contact in the Patagonia Mountains. Additionally, we have been concerned with levels of genic heterozygosity in populations, and with degrees of genetic similarity among populations of T. bottae. Information on genetic variation in pocket gophers is of particular interest because of the propensity of these rodents to differentiate microgeographically, presumably as a result of strong geographic subdivision of populations, low vagility, and other specializations of behavior and ecology associated with the fossorial niche (Thaeler, 1968; Patton, 1972a). Such differentiation has been amply documented by both morphologic (Hall and Davis, 1935; Durrant, 1946) and chromosomal (Patton and Dingman, 1970; Patton, 1972a) studies of T. bottae. MATERIALS AND METHODS One-hundred and twelve gophers, including 74 T. bottae, 30 T. umbrinus, and eight individuals idelntified as F1 hybrids on the basis of their chromosome complemerits were studied. This material includes animals of both species from the zone of hybridization in Sycamore Canyon, Patagonia Mountains, Arizona, and four samples of T. bottae from localities well removed from the contact zone (Fig. 1). Individuals were prepared as museum specimens and are deposited in the Museum of Vertebrate Zoology, University of California. Localities and sample sizes are